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In a network with a dual-redundant link between two switches, will any data be lost during the switchover from an active link to the blocked link when the active link fails?

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It is possible, even likely, that at least one (probably more) frames will be lost on any failover.

The speed of the failover is highly dependent on what type of redundancy you are using. Spanning Tree is the slowest, routing is an order of magnitude faster, and etherchannels are yet and order of magnitude faster than that.

When frames are lost, there is nothing in layer-2 to request that they be resent. If the data in the frames is TCP, the upper layer will request that the packet containing the TCP segment be resent. UDP does not do that, so it would be up to the application to request a resend if it is built that way.

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If i remember, default spanning tree (802.1d) takes about 50 secs to reconverge. That is the duration for which you will be losing frames.

  • A link is detected down by the loss of 10 hello pkts. These are usually 2 secs apart. So total=20 secs. This is called the max-age timer
  • Next it has to transition another blocked port through listening and learning states into a forwarding state. This transitioning takes 15 secs for listening state, and 15 for learning. So total=30 secs. These are called the forward-delay timers.

Sure, you can modify these timers to make is quicker. Or use RSTP(Rapid Spanning tree), which is more like the defacto now. RSTP is much quicker, as takes just a few secs to reconverge.

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  • Good answer Ajay, as an addenum to that: Convergence happens when all ports on bridges and switches have transitioned to either the forwarding or blocking modes. The ports on a bridge or switch running STP can transition through five different states. When the network needs to re-converge, the Listening state takes about 15 seconds, followed by the Learning state that usually takes another 15 seconds, and 20 seconds to change to Forwarding, Blocking or Disabled. If no timers have been changed the total time of convergence is about 50 seconds.
    – Ty Smith
    Commented Oct 30, 2016 at 6:31
  • Three advantages of RSTP: link status of each port are monitored pro-actively (instead of waiting for the BPDU messages) to detect network topology changes. RSTP is backward compatible with STP switches. The Switch ports that participate in RSTP have three states – Discarding (Does not accept/ forward any data but listens to BPDU messages), Learning (Once the network topology change is detected/ activation request comes via the BPDU message and filtering/ forwarding table creation is initiated) & Forwarding (RTSP ports start accepting and forwarding data packets/ frames).
    – Ty Smith
    Commented Oct 30, 2016 at 6:40

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